Some embodiments relate to a method of calibrating a patient monitoring system for monitoring the location of a patient during radio therapy. In particular, some embodiments of the present invention concern a method of calibrating a stereoscopic camera system for use with a radiotherapy treatment apparatus and the like where accurate positioning and the detection of patient movement is important for successful treatment.
Radiotherapy involves projecting onto a predetermined region of a patient's body, a radiation beam so as to destroy or eliminate tumours existing therein. Such treatment is usually carried out periodically and repeatedly. At each medical intervention, the radiation source must be positioned with respect to the patient in order to irradiate the selected region with the highest possible accuracy to avoid radiating adjacent tissue on which radiation beams would be harmful.
When applying radiation to a patient, the gating of treatment apparatus should be matched with the breathing cycle so that radiation is focused on the location of a tumour and collateral damage to other tissues is minimised. If movement of a patient is detected the treatment should be halted to avoid irradiating areas of a patient other than a tumour location.
For this reason a number of monitoring systems for assisting the positioning of patients during radiotherapy have therefore been proposed such as those described in Vision RT's earlier patents and patent applications U.S. Pat. Nos. 7,889,906, 7,348,974, 8,135,201, US2009/187112, WO2014/057280, and WO2015/008040 all of which are hereby incorporated by reference.
In the systems described in Vision RT's patent applications, stereoscopic images of a patient are obtained and processed to generate data identifying 3D positions of a large number of points corresponding to points on the surface of an imaged patient. Such data can be compared with data generated on a previous occasion and used to position a patient in a consistent manner or provide a warning when a patient moves out of position. Typically such a comparison involves undertaking Procrustes analysis to determine a transformation which minimises the differences in position between points on the surface of a patient identified by data generated based on live images and points on the surface of a patient identified by data generated on a previous occasion.
Treatment plans for the application of radiotherapy are becoming increasingly complex with treatment apparatus having multiple or floating iso-centres. Also, there is an increasing trend to make use of higher doses of radiation during treatment in order to reduce overall treatment time. Such increasing complexity and higher dosages bring with them the increasing possibility of mistreatment. There is therefore an ever increasing need for improvements in the accuracy of patient monitoring systems.
To obtain a reasonable field of view, in a patient monitoring system, cameras monitoring a patient typically view a patient from a distance (e.g. 1 to 2 meters from the patient being monitored). Vision RT's patient monitoring systems are able to generate highly accurate (e.g. sub millimeter) models of the surface of a patient. To do so, it is essential that the monitoring system is calibrated in order to establish camera parameters identifying the relative locations and orientations of the image capture devices/cameras, any optical distortion caused by the optical design of the lens of each image detector/camera e.g. barrel, pincushion, and moustache distortion and de-centring/tangential distortion, and other internal parameters of the cameras/image capture devices (e.g. focal length, image centre, aspect ratio skew, pixel spacing etc.). Once known, camera parameters can be utilised to manipulate obtained images to obtain images free of distortion. 3D position measurements can then be determined by matching corresponding portions in images obtained from different locations and deriving 3D positions from those matches and the relative locations and orientations of the image capture devices/cameras.
Typically, calibration of a patient monitoring system involves capturing multiple images of a calibration object of known size and with a known arrangement of calibration markings at various orientations and various locations within the field of view, and processing the images using information regarding the expected locations of the markings on the calibration object to determine the various parameters. The accuracy of the calibration then very much depends on the number of images used in the calibration process. The greater the number of images used, and the greater the variation in the orientation of the calibration plate between the various images, the more accurate the results. For example, the HALCON machine vision software package from MVTec Software GmbH requires at least 10 to 15 images of a calibration plate, and may require significantly more than this if the calibration object is small relative to the field of view of the image capture devices/cameras. As a consequence of the number of images that are required to be captured, it can be a time consuming to calibrate a computer vision system using such processes.
When a computer vision system is used for monitoring the positioning of patients during radiotherapy treatment, a system needs to be re-calibrated very frequently (ideally for each individual patient treatment session) to ensure that the parameters used to process captured images and generate computer models accurately reflect the current relative locations of the stereoscopic cameras. The need for high accuracy and regular re-calibration is exacerbated in the case of patient monitoring system where patients are viewed from a distance as very small changes in the orientations of the cameras can have a significant impact on the accuracy of models. Due to this sensitivity very regular calibration is particularly important for patient monitoring systems particularly where cameras may accidentally be knocked or change orientation, for example in areas that are prone to earthquakes such as California or Japan small earthquake tremors could cause movement of the image capture devices/cameras of the system leading to errors in patient positioning and treatment. However, where such calibration is undertaken at the desired frequency this has an adverse effect upon the throughput of patient treatment.